Colored mirror



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WAVELf/VGT/l IN M/lL/M/CRQ/VJ INVENTORS WILLIAM H. CO LBERT ATTUR/VIVJ' Patented Aug. 22,1950

COLORED MIRROR William K. Colbert Breckenridge, and Willard L. Morgan, 'Haveriord, Pa., assignorl to Libbey- Owens-Ford Glass Company, Toledo, Ohio, a

corporation of Ohio Application February 11, 1946, SerialNo. 848,875

Our invention relates to colored mirrors. It has to do more particularly with the production of mirrors having desired color and reflectivity characteristics. More specifically, it relates to the production of mirrors of desired color wherein the color is obtained primarily by producing interference of light rays which strike' the mirrors.

This application is a continuation-in-part of our copending application Serial No. 478,076, flied February 16, 1943, now Patent No. 2,394,533.

Despite the wide possible use of colored mirrors in furniture, store decoration, theater decoration and other decoration, sales displays, etc, and as automotive mirrors, and the possible use of colored reflective surfaces to add to the attractive-' ness of shaped glassware, there has been little use to date of such mirrors and surfaces due to the expense of producing them and the few shades available.

The accompanying drawings will aid in the understanding of our invention.

In the drawings:

Figure l is a diagram showing spectral refl tion curves for silver, gold, copper and lead sulflde mirrors of the prior art;

Figure 2 is a diagram illustrating light waves of a single ray of a definite color;

.Figure 3 is a diagram illustrating light waves of two rays of the same type vibrating in the same wave phase;

Figure 4 is a view similar to Figure 3 but showing the rays vibrating in opposite phase;

Figure 5 is a diagram illustrating how various light rays will be reflected from a reflecting layer;

Figure 6 is a diagram similar to Figure 5 but illustrating a combination of reflecting layers according to our invention; v

Figure 'I is a transverse vertical sectional view of a second surface mirror made according to one form of our invention;

Figure 7a is a transverse sectional view illustrating a first surface mirror made according to one form of our invention;

' Figure 8 is a diagram illustrating how various light rays will be reflected from a combination of multiple reflecting layers according to our invention and showing some of the paths followed by the light rays;

Figure 9 is a transverse vertical sectional view of a second surface mirror made according to our invention employing multiple reflecting layers according to this invention;

Figure 9a is a similar view of a second surface mirror niadein accord with the invention em- 2 ploying numerous partially transparent reflecting layers of different materials and backed by an opaque reflecting layer;

Figure 10 1s a transverse sectional view illustrating a flrst surface mirror made according to our invention which employs multiple reflecting layers;

Figure 10a. is a similar view of another mirror construction in accordance with the invention in which some of the multiple reflecting layers are found upon each side of the support;

Figure 11 is a view similar to Figure 9 in which numerous partially transparent reflecting layers of two different materials are employed in the mirror in spaced relationship in accordance with the invention Figure 12 is a diagram illustrating for comparison a spectral reflection curve of a simple semi-transparent fllm of lead sulfide and a spectral reflection curve of our compound mirror made by using a semi-transparent fllm of lead sulflde of comparable thickness plus an opaque backing-up film of silver;

Figure 13 is a similar view comparing the effects produced by a simple semi-transparent lead sulflde fllm of a different thickness than that of Figure 12 with the combination of a similar thickness of such fllm and an opaque silver film;

Figure 14 is a diagram illustrating 'the colors of our interference'mirrors when these are prepared with an opaque silver reflective backing and cooperating layers of copper and silica or of copper and magnesium fluoride in various thicknesses;

Figure 15 is a diagrammatic spectral reflection curve such as isfound with our mirrors employing a single partially transparent reflecting fllm approximating one quarter wave length in thickness;

Figure 15a is a similar diagram as found with 'mirrors'employing a partially transparent reflecting fllm or cooperating films in which the optical thickness is several quarter wave lengths; Figure 16 through Figure 19, inclusive, are

spectral reflection curves for mirrors made under the invention as described more fully below.

' Silver mirrors of 88-92% reflectivity value made with clear ordinary plate glass have no color, since all the various light rays are reflected approximately equally, as seen from the spectral reflection curves in Figure 1. Some colored mirrors have been used commercially which were made by silvering colored glass, the color thereby secured being the color of the glass. The color of the glass arises from the fact that the glass 3 absorbs some types of light rays more than others and the light rays which are transmitted, with the least absorption, thus impart the color to the glass. Thus, Solex green glass, made by Pittsburgh Plate Glass Company, is green because it absorbs much of the red and blue light out of the white daylight as such light passes through the glass. A green mirror, made with silver on the SoIex green glass, showed a reflectivity value, for visible light, of 61 and similar mirrors, made with a glue glass and a flesh colored glass, showed reflectivity values of 35 and 68.3%. These are all lower than the reflectivity value of silver on the colorless glass. by reason of the loss of the colored light, which the colored glasses absorb. The use of such colored glasses is expensive and, moreover, satisfactory quality for forming mirrors is not readily available.

Using plain plate glass, some colored mirrors have been made in which the color arises from the selective reflection of the various light waves of different colors to different degrees. Thus, gold mirrors show a spectral reflection curve, as shown in Figure 1. The gold mirror appears yellow because little green or blue is reflected while much larger amounts of yellow and red are reflected, the overall reflectivity of total visible light for gold mirrors being about 75%. The copper mirror, which reflects about 55% of all light, is orange-red, being different from the gold mirror in that most of the light reflected is red. Copper mirrors have not been commercialized due to the unreliable methods for their production. Gold mirrors are expensive and, therefore, have not gone into wide use.

The use of fuchsin or methyl violet dyestuffs in opaque layers for mirror surfaces has been suggested. Such mirror surfaces reflect 11.6% of the total visible light. Experience with such mirrors indicates that where such dyes are used as the reflective layer they must be present in fairly thick opaque layers and the reflectivity percentage is always low and the mirrors quite dark. The colors secured are invariably the complementary color to the normal color of the dyestuff when seen in solutions by transmission. Thus, methyl-violet gives a green mirror s nce the fllm transmits red and violet light and reflects, se lectivelv, the green light which it does not transmit. Thus, the color in these mirrors arises solely by selective reflection, just as is the case with gold, wh ch when viewed by transmission is green, the gold being relatively transparent to green light but opaque to the red and yellow light which it reflects selectively. Mirrors of this type are not stable, the films rapidly breaking up and spotting and in service the color soon changes to muddy non-reflective grays. As a result they have had no commercial use in spite of the demand for colored mirrors.

Mirrors made with platinum, iridium, or aluminum are silvery in appearance and without color,

while those made with chromium, silicon, or lead sulfide are dark and without color tone. Likewise, glass coated with asphalt or black paint, with reflectiviln values of 5%, and mirrors of black opaque glass, with 5% reflectivity, are not very useful because of their extremely low reflectivity values and the very dark images which thus appear in such reflective surfaces.

Thus, despite the wide possible use of colored mirrors and colored mirror surfaces, there has been little use to date because of the expense of producing such mirrors and surfaces and the few colors and shades available, as shown above.

One of the objects of our invention is to provide colored mirrors or other mirrored surfaces of a wide range of color characteristics and of a wide range of reflectivity percentage characteristics which can be controlled as desired.

Another object of our invention is to provide mirrors or other mirrored surfaces of various colors and reflectivity values in which the colors are mainly secured by light interference effects and are permanent and inexpensive.

A further object of our invention is to provide mirrors or other mirrored surfaces of various colors and reflectivity values in which the colors are secured by light interference effects produced by two or more extremely thin partially trans- I parent layers of different substances acting cooperatively to produce interference color effects, and in which the color and reflectivity of the mirror or article are further modified by an opaque mirror reflective backing layer.

Various other objects will be apparent from the following description.

The colors which appear in thin-walled soap bubbles and in very thin oil films do not arise from any inherent color in the soap film or in the oil. Also, in these cases, it is well recognized that the colors do not come from any selective color light absorption effects, as the soapy water and oil do not show such colors directly. As the soapy water and oil in bulk are also clear and transparent and non-reflective, it is apparent that the colors do not arise from any selective reflection of light. The colors are known to occur from interference of the light rays, which results in a neutralization or loss of certain colored lights and the residual light which then appears is, obviously, colored. The particular color of light rays removed by interference is dependent upon the thickness of the film and its refractive index, as will be shown later. It is well known that interference colors can only appear in extremely thin films which are of a thickness comparable to one-fourth the wave length of light and which are at least partiall transparent.

We have found that by depositing reflective coatings of various materials in extremely thin films which are still partially or considerably transparent that the two or more extremely thin semi-transparent laminae of different substances will act cooperatively to produce light interference colors and upon backing these with a substantially opaque reflective coating of another material, we can secure a wide range of colored mirrors of various reflective characteristics for the various rays of light and in terms of total light reflection. We obtain these effects by simple and inexpensive means and colored substances or colored glass are not necessary. The colors are permanent and do not fade or alter, as they are dependent primarily upon physical light interference effects. The substantially opaque reflective backing need not be colored although it may be, but it is of a different refractive index from any immediately adjacent contacting layer of semi-transparent reflective material and by its substantial opaqueness it prevents viewing of the background through the mirror.

We have found that by the controlled deposition of very thin uniform semi-transparent films of many materials we can secure reflecting bodies of a wide range of color and reflectivity characteristics. The material used as the reflecting substance in any of the layers in the products need not have any inherent color. Such mirror bodies have been described more particularly in our copending applications Serial Nos. 473,473 and 473,474, flled January 25, 1943, and in Serial No. 476,076, flied February 16, 1943. By backing the composite of the semi-transparent mirror layers with a substantially opaque or semi-opaque mirror layer of a material of good reflectivity, which may or may not be colored, we produce by the cooperative action of the various reflective layers entirely new and different mirrors with characteristic color and reflectivity properties. Dther combinations of reflective layers-may be employed according to our invention. For example, we may employ semi-transparent interference layers, backed up by a transparent layer, which may have color, and which is backed up by a substantially opaque mirror reflective layer. Any desired number and arrangement of layers upon a support such as glass may be provided as long as at least two semi-transparent interference color producing layers are present in the structure in immediate continuous contact as a composite and the mirror structure is backed up by at least one reflective layer which is opaque or sufficiently opaque to prevent viewing of the background.

The development of colors by light interference has been explained upon the basis that light radiations are of a wave form. such as shown in Figure 2, which represents a single ray of a deflnite color. Blue light diil'ers from red in that the length of the waves is shorter, in the case of the blue, and longer, in the case of the red. The other visible colored rays of light are of intermediate wave lengths. White light is composed of a mixture of all of these visible rays. If two rays a and al of the same monochromatic type or wave length happen to be vibrating in the same wave phase, as in Figure 3, they amplify each other and the intensity is increased. However, if they happen to be vibrating in opposite phase, as shown at a2 and 0-3 in Figure 4, they interfere with or oppose each other and a loss of light intensity results. Thus, if in some way we can make some of the blue rays in ordinary white light get out of phase with other blue rays of the same wave length, we can remove some of the blue from the ordinary light. The remaining light will then no longer be white but of 'a color resulting from the remaining green, yellow, orange and red rays and will appear a reddish-yellow color.

If we consider two light rays impinging upon a reflecting substance 3, as in Figure 5, and assume that ray b is reflected at the top surface c while ray 1! passes on through the semi-transparent layer s to the bottom surface e before it is reflected, it is apparent that the second ray has had a longer path to travel before it again emerges from the top surface c of the layer s. Thus, the ray (1 lags considerably behind the ray b and in consequence, the crests and troughs of the waves of the two rays d and b can be arranged so that the difference in phase is such that interference of the waves of two of such rays, entering or being reflected at any point on the surface of s, will occur. The time difference between the waves of the two rays will be dependent upon the thickness of the layer s and the speed with which the given light ray travels in the material comprising the layer s.

As the number'of complete wave cycles which any given monochromatic light ray makes per second or its frequency is a flxed constant, the variation in speed of travel of the light ray in diflerent media causes a shortening or lengthening of the actual length of a wave as it travels through the various media. Wave lengths for light are generally given with reference'to their values in traveling through air and the speed of travel for all light rays in this medium is given as 299,910,000 meters per second. i In denser ma-- terials, the light rays move slower and all the light rays do not necessarily move at the same speeds. The proportionality constant N between the velocity of light in a given substance and the velocity of light in air is called the refractive index for that substance.

N: Velocity in air wave length in air Velocity in substance wavelength in substance If by it we indicate the wave length in air and by X. the wave length in some other'substance, it is apparent that these are simply related as follows:

N varies somewhat with different monochromatic waves of diflerent wave length but a similar equation holds for each wave length considered. In general, as the variations are usually small, a single constant for N can frequently be applied for all waves in the visible light range.

In order for the ray d to come out of the top surface and be degrees out of phase and to thus interfere with the ray b, it must be slowed down in time and distance equal to the distance of one half of a wave length of the ray in air, i. e.

As the ray is traveling only UK as fast in the substance 3, comprising the layer, and must traverse the thickness of the layer twice, the thickness of 3 required to cause an equivalent slowing effect is then In a similar way, thicknesses equal to 1, 3, 5 or any uneven integral multiples of this quantity should also show interference eifects. If the ray d is reflected within the layer s not once but two times, it is apparent that the film needed for interference need be only thick. Thus, as shown in'-Figure 5, wherein the ray I is shown as an inclined ray striking the surface of the layer the light ray is reflected twice within the layer. If the ray is reflected any number of times, such as R times, then the film needed is thinner and is of a necessary thickness as given by 4N.R Furthermore, it is apparent that similar thicknesses equal to 1, 3, 5 or any uneven integral multiples of such quantities will show interference effects with rays which are multiply reflected within the layers. Thus, the suitable film thicknesses, for our films or composites of contacting partially transparent fllms are of the order of one-fourth of a wave length of any visible light ray or some small multiple or submultiple of this, divided by the refractive index of the material used in the semi-t fllm or films.

Since the various colored rays of light have different wave lengths and these range from 4000 to 7500 Angstrom units or 0.4 to 0.75 micron or thousandth millimeters in the visible spectrum, it is obvious that a semi-transparent film which is thick enough to cause interference with the short blue rays will not cause interference with the long red rays, etc. Thus, each thickness of semi-transparent fllm will take out certain defined portions of the spectrum and a film will take on a series of different colors as the thickness is varied. As will be shown in the examples which follow, the colored mirrors of our invention produced by light interference, show varied colors, depending upon the thickness of the semitransparent mirror film or films employed. In the spectral reflection curves for these mirrors, the portion of the curve and minima of reflectivity caused by interference, shifts from the blue range of wave length toward the red, as the semitransparent film or films are made thicker.

In Figure 15 there is shown a generalized diagram of the spectral reflection curve type found with our mirrors when a single semi-transparent mirror film approximating a one-quarter wave length of light thickness la or a group of two or more such semi-transparent mirror films are employed cooperatively to approximate a one-quarter wave length of light in optical thickness; the mirrors in each case being backed by an opaque mirror reflecting layer. The location of the minima shifts with increasing thickness as indicated above, the minima being clearly the wave length of light removed by the light interference effect and the wave length to be considered in establishing the optical thickness of the semi-transparent mirror or reflective film or similar multiple cooperating films. As the total optical thickness of the single semitransparent film or semi-transparent cooperating films becomes a multiple of a quarter wave in thickness such as 3 or 5 the distance between adjacent minima in the spectra becomes smaller as shown by the generalized diagrammatic spectral reflection curve for mirrors of such type made under this invention as shown in Figure a.

The semi-transparent film or films must be of very uniform thickness, if the color is to be the same throughout the mirror. This has called for special means of producing such semi-trans parent reflective layers in view of the extreme uniformity and extreme thinness of the semitransparent layers desired. On the other hand,

it is within the scope of our methods to produce colored mirrors of mottled or variegated colors where the semi-transparent film thicknesses of one or several of such films are deliberately varied to cause such effects.

Interference effects in perfectly transparent materials or in cooperatively acting multiple layers of these in immediate contact do not occur at thicknesses beyond about the ninth multiple of the quarter wave length factor already described. In semi-transparent materials, the increasing absorption of light by the increasing thickness of film, which is exponential with respect to the thickness, may soon leave so little light reflected from the bottom surface e that no interference effect can be found in the reflected light which is then coming entirely from the top surface 0. Obviously if a mirror film is opaque all of the light is absorbed before ever striking the bottom surface 2 and therefore no light is thrown back to cause interference effects, particularly as the filmobviously must be traversed twice ii interference is to be obtained. Since most commercial mirrors have been. made with mirror layers thick enough to be opaque, they have not shown any interference colors and their entire reflection occurs at the top surface of the mirror layer. The occurrence of interference by multiple reflection within the layer or layers, as shown in Figure 5 with ray j, and in Figure 8 with rays 11., p, q, o, and t originating from light ray 7', is very quickly limited by the transmission values for the film or films and, in practice, we

have not found evidence for more than two to four such internal reflections although more may occur in the more highly transparent films and in the thinnest of the films.

It is thus apparent that the amount of light which comes back through our semi-transparent mirror film or films and is then available at the upper surface or intermediate surfaces to cause light interference effects is a function of the transparency of the material or materials used in the semi-transparent layer or layers, the transparency being considered with respect to the particular wave length of light at which interference is developed as determined by the film thickness or cooperating film thicknesses. The transparency of the film or fllms with respect to ordinary visible light generally is thus important and we use films which are partially or semi-transparent or which show a visible light transmission of 10 to 90% in the thickness employed. Preferably at least one of the partially transparent reflective films should have a transparency in this range and the other or others in a laminae of cooperating layers of different substances may have even higher transparency. Where all the partially transparent thin films are of over 90% transparency we can still prepare our colored mirrors but the mirrors are of low color saturation.

The amount of light reflected from the surface of a single semi-transparent layer is a function of the refractive index, being greater the larger the refractive index for the substance comprising this film, and we find it also becomes greater as the thickness of the semi-transparent film increases until it is opaque. While it may thus be an advantage to use a material or materials for the semi-transparent mirror layer or layers which have a high refractive index to secure greater brilliancy of reflection and to permit the use of thinner, more transparent films, thus giving greater efiiciency of light removal by interference and thus giving purer and deeper color tones, we do not restrict our selves to the use of any particular range of refractive index materials but may use a wide range of substances. It is apparent that a material of about 50% reflectivity value, when viewed in a normally opaque thick film, which can be laid down in very thin films which are of high transparency, will show the brightest and deepest interference colors as mirrors. We may, however, use materials which in their ordinary opaque film or in bulk show much higher or lower reflectivity values than this and are not restricted to any range in this constant, although values lying between and 20% are preferred. Thus, thin calcium fluoride coatings will reflect something less than 10% of the light at the top surface and are very transparent (approximately 96-97 and the reflected light coming from the back surface causes interference colors to develop but the depth of color resulting is low due to the white light mixed with the colored light being of a high intensity. It is neces-' sary that the film or films used for our mirrors have the characteristic of giving some specular or mirror type reflection of light, since diffuse type reflection is not satisfactory. As shown by the calcium fluoride this reflected specular type reflection of light can be quite small from a given layer.

The statements of the preceding paragraph have discussed the usual reflectivity of a single film of material in contact with air and the relationship of such' reflectivity to the refractive indexof the material comprising the film. The reflectivity (R) at perpendicular incidence of light of a layer of material in contact with another continuous medium at the interface between the two media is known by optical principles to be related to the refractive indices (N1,N2) of the two media as follows:

As the rerfactive index of air is l the relationship simplifies for a single film of continuous nature as discussed above. Withthe partially transparent films of solids of continuous nature employed in contact with each other in forming our mirrors it becomes apparent that the reflectiviiw conditions applying at the various interfaces between such and at any interface between these and or the support for the mirror and with the opaque specular or mirror reflecting backing are determined to a large degree by the refractive indices of the two adjacent contacting materials or films. While it is often of some advantage that these be of considerably different refractive index, materials of closely similar indices may also be employed.

If adjacent layers of different materials are of the same refractive index then no reflection occurs at the interface between such layers and the. two layers will cooperatively act as a single layer of such refractive index and a ray of light will actin such two layers as shown for the single layer s in Figure 5. The limiting light absorption of the composite is then a joint result of the individual absorptions of the layers. While some or all of our adjacent partially transparent reflective films of continuous nature and of different substances may be of similar refractive index and thus act cooperatively in such manner, we generally employ such adjacent contacting and cooperating films of different materials having different refractive index. It is necessary however that a partially transparent reflective .fllm contacting a surface of the support, such as a glass plate, be of different refractive index from that of the support if such fllm is not to become optically a part of the support. on the other hand where the film contacting the support and the support are of the same index of refraction and where there is also prment one or more other partially transparent layers of different refractive index and of the desired range of thickness in the mirror structure and an opaque mirror of refraction of such backing layer be diflerent from the index of refraction of the contacting partially transparent reflecting layer or the index of refraction of the contacting support mate. rial. Thus, the backing opaque reflective layer must be of a different refractive index from the substance it contacts with in the various types of mirror structures under the invention: Consideration will show that where the normally reflective opaque layer is used in a structure in which it has the same index of refraction as the layer or support in direct contact with it that such normally reflective layer would show no specular reflection value at such interface. The

light would then penetrate into such opaque layer and be absorbed instead of being reflected and'the layer would act for all purposes as a non-specular reflective opaque absorbing back- 1118.

Colored mirror structures utilizing a layer or multiple layers of thin partially transparent reflective films cooperatively and with an opaque non-specular reflecting backing are the subject of copending applications Serial No. 473,473, filed January 25, 1943, (now patent No. 2,430,452) and the continuation-in-part from the same, Serial No. 646.876, filed February 11, 1946.

As will be apparent from the drawings representing our mirror structures and from the above discussion our thin partially transparent reiiective films are layers of materials in a continuous solid form such as can optically act as a layer or layers and cause interference and they are in no case coatings made up of small sepmate isolated discrete areas or spots covering only a part of the contacting'surface such as might cause color phenomena by diffraction of light.

We flnd that when our thin semi-transparent reflective films, in which interference colors are developed, are backed up with a relatively thick semi-opaque or substantially opaque film of a mirror material that light, which is normally lost an opaque mirror body 1', as illustrated in Figure reflecting layer backing is employed. colored 6, we obtain a reflection of. a considerable portion of the light represented in rays g and h in Figure 5. This results usually in increased general light, reflectively of the mirror or mirrors as a whole which includes such a combination of layers. Furthermorafor example, if the semitransparent mirror film or cooperatively acting laminae or layers of such multiple films is of a thickness that the reflected blue rays, for example, are out of phase and are, therefore, destroyed by interference, this increased reflection for blue light at surface e results in more light for interference at the top surface 0 and greater destruction of blue light. Thus, the red color of the mirror is also intensified. For this to happen it is not only necessary that the semi-transparent fllm or illms be fairly transparent to blue light but that the relatively opaque reflecting body 1' reflect blue light. If the semi-transparent layer mirrors of higher reflectivity are thus secured by the use of a substantially opaque mirror fllm in back of our semi-transparent mirror layers, and a wide variety of colors and refiectivities can be secured in our mirrors.

Where the semi-transparent reflective film or group of such films is selectively more transparent to some colors or specific wave lengths, such as red in the case of lead sulfide alone or with silica, and green in the case of gold alone or with magnesium fluoride we find that by backing up our very thin reflective films with a thick substantially opaque film of a mirror body, such as silver particularly which is a good reflector for all colors of light as shown in Figure 1, these rays are then returned out through the face or front of the mirror with consequent higher refiection for these particular rays. Thus, while the mirrors made with extremely thin lead sulfide semi-transparentcoatings or such cooperating films of lead sulfide and silica together, which have been backed up by forming a substantially opaque silver mirror layer over the lead sulfide, show the spectral range of colors, depending upon the thickness ofthe lead sulfide or of the cooperating films used and these mirrors refiect more light and are brighter, in consequence, and are also of definitely redder tones, excepting where interference occurs in the red, than the similar films having only the semi-transparent lead sulfide coating or cooperating lead sulfide and silica coatings. With thin semi-transparent gold mirrors backed by substantially opaque silver, the mirrors are also mostly brighter and of higher reflectivity in the green than the mirror films having the gold coating only. The same change occurs with the use of gold coatings backed by magnesium fluoride when such combined films are backed, for example, with an opaque silver or aluminum mirror reflective coating.

We may use many other substantially opaque mirror film's, other than silver, behind our semitransparent thin film mirror layers or composite layers of semi-transparent thin films for the purpose of modifying the general hues and refiectivities of those layers and of thereby securing our new colored mirrors. In general, any material giving a specular mirror reflectivity, rather than the diffuse type reflectivity, andof a total reflectivity of over 15% has been found to be useful as a substantially opaque mirror backing layer for use with the thin semi-trans parent reflective films and composites of such films, in forming our compound mirrors described. The high general reflectivity for all types of visible light make silver and aluminum with 90% refiectivities, magnesium with 73% reflectivity, platinum with 63% reflectivity, iron with 55% reflectivity, nickel with 60% reflectivity, rhodium with 75% reflectivity, as well as cobalt, chromium, iridium, indium, antimony, molybdenum, palladium, cadmium and other silvery appearing metals, quite useful as backing reflectors. We may also use opaque lead sulfide films of 30% refiectivity, as shown in Figure 1, or molybdenum or antimony sulfides of about 40% reflectivity, or

. 12 other metallic sulfides or-minerals having a metallic reflection.

Mirror materials of selective color reflectivity characteristics are of value as backing reflectors in combination with the semi-transparent interference color reflective films in that a further means of modifying the color characteristics of the compound mirror, thus formed, is secured. Thus, we may use a substantially opaque relatively thick layer of gold or of pyrite on the back of a thin semi-transparent layer of lead sulfide or upon any of our composite partially transparent contacting laminae of a plurality of layers which show interference colors. In Figure 1 while gold shows about 77% reflectivity, the higher selective reflection of red and orange light by ordinary opaque gold mirrors is quite apparent. Copper and its oxide cuprite both reflect strongly in the reds. Silicon and selenium. reflect more strongly in the blue than in the remaining visible light, the reflectivity ranging from 40% in the blue, in each case, down to 28 and 25% in the red, in the respective cases.

It is necessary that the material comprising the backing mirror be of a diflerent material and as already discussed of a different index of refraction from the substance comprising the very thin semi-transparent layer in contact with such so that a definite light reflective surface be formed on the front of the backing layer. In giving specular mirror reflectivity the materials chosen for our backing layers are such that images clearly visible to the eye are apparent therein.

As a material suitable for the forming of the very thin semi-transparent reflective layers, we have found the use of very thin films of lead sulfide to give particularly attractive results. In

its use in the normal opaque mirrors of fairly thick films, it is a. practically colorless mirror, as shown by Figure 1, which shows the reflectivity, about 30%, for all the wave lengths of light to be about the same. It has a high refractive index of 3.9 and is quite transparent in the thicknesses which will cause interference effects. Gold, having a refractive index of 1.18 at 4400 Angstroms and of 0.47 at 5890 with a normal reflectivity curve, as shown in Figure 1, is quite transparent in very thin films to green light. So also is copper which reflects, as shown in Figure 1, when in opaque films and which has a refractive index of 1.10 at 5000 Angstroms and 0.44 at 6500. Both of these substances, either alone or in combination with partially transparent layers of other substances, may be used in providing mirrors of a range of colors, when used in films which are semi-transparent and which are sufflciently thin to cause color development through light interference efiects. Other sulfides of a metallic luster, such as stibnite and molybdenite, having a refractive index of 4.3, and each of about 40% general reflectivity in the visible range with a slight bluish cast, are quite suitable. Pyrite, which reflects a maximum of in the red and a minimum of 45% in the blue, may be used as a single partially transparent film or as a film in a composite element comprising a plurality of thin partially transparent films, as may also silicon, normally of about 38% reflectivity, and having a refractive index of 3.8 to 4.2. Antimony, having a refractive index of 1.62 and a reflectivity of about 55%, can likewise be used. Fluorite or calcium fluoride, having a refractive index of 1.43, and other fluorides of about the same refractive index, such as magnesium fluoride having an index of 1.88, may be used as a reflective layer, although these very transparent substances are of low reflectivity values as previously mentioned. Thus, for fluorite alone the estimated reflectivity value would be 3 to 4% uniformly throughout the visible range and very thin films of this material give low reflectivity values and give light interference tints where used as separate single films.

It is also possible to use films, which are in the desired thin range and which cause'interference coloration of mirror type reflectors in which the film is a jointly deposited mixture, chemical combination, or alloy of film-forming materials. For example, a jointly deposited mixture of gold and lead sulfide is suitable.

While no colorneed be present in the'material or materials used as the semi-transparent reflecting layer or layers, such as in the case of lead sulfide, the use of such materials as gold, showing selective specular reflection, as the semitransparent layer, brings an additional source of possible variation of both the hues and spectral reflectivity characteristics in our final compound mirrors produced by using these layers backed by a substantially opaque reflective layer. The choice of a material for the semi-transparent reflective film which has color characteristics in its normal reflection, such as gold, imposes its normal reflection spectral limits, to some degree, on the general nature of the light reflected by the film and from which various spectral components are subtracted by the light interference effects, depending on the thicknessof the fllm usedand its refractive index. Thus, in general, our compound mirrors made with a very thin semi-transparent layer of gold film or of such gold and magnesium fluoride fllms are of higher total reflective values and thus brighter, and also of particularly higher reflectivity in the yellow and red, than are similar compound mirrors having thin semi-transparent layers of lead sulfide, or of lead sulfide and silica, the colors and reflectivities, in each case being derived, to a main degree, by the interference effects arising from these layers in combination with the normal re flective characteristics for these materials and, in turn, further modified by the nature of the substantially opaque mirror backing used. Cuprite or cuprous oxide, which is a bright red and gives a reflectivity in bulk of about and has a refractive index of 2.7, as well as ordinary cupric oxide have been found to be quite satisfactory as materials for use in the semi-transparent layers in making our compound colored mirrors.

While silver or aluminum alone as a single layer do not lend themselves to giving colored reflective mirrors. due to failure of light interference to occur in these'single films, we have found, when employed in mirrors in very thin partially or largely transparent layers with other thin films of different substances, that excellent interference colored mirrors are obtained when a suitable opaque backing is applied to such structures. Other silvery metals, such as those already described 'as suitable for .the backing opaque reflective layers may alsob used in forming the thin partially transparent layers which either singly or in combination with other metals or materials in multiple composite layers of such nature are employed in forming our mirror structures. Thus, for example, platinum, iron, or cobelt or even the darker metals, such as chromium,

are suitable. Most oxides, sulfides, selenides, tellurides, and halides of inorganic nature may be employed informing the thin partially transparent reflective layers in our mirrors and by way of example, but not by limitation, we may add to such substances already enumerated, silica, lead oxide, magnesium oxide, silver oxide, aluminum oxide, arsenic oxide, tin oxide, titanium oxide, iron oxide, zinc sulfide, copper sulfide, lead selenide, lithium fluoride, silver chloride, sodium chloride,lead chloride, and lead fluoride. Most any solid inorganic material ay be used and we may also use similar thin p tially transparent layers of solid organic materials, and particularly of plastic materials such as the polymethacrylate resins. The commercial polymethacrylate plastics have a refractive index of approximately 1.49.

The thin partially transparent layers may be produced inseveral ways to secure the necessary uniformity and continuity of thickness in our semi-transparent reflective films and thereby secure uniformity of color and reflectivity characteristics throughout a mirror body and at the same time permit control of the forming of the desired thicknesses in such films. Deposition from a solution as a support is withdrawn from such solution at a constant rate is a well known method applicable to the organic materials generally and suitable for such other simple inorganic substances as are readily soluble. Where the mirror films are deposited chemically, the

deposition reactions must be greatly retarded, as compared with former operations. Thus, the

reaction mixtures and temperatures of deposition must be changed toward slowing down the entir deposition process so as to give more uniform and even development of crystal nuclei and even slower than normal rates of growth onto these nuclei. This deposition process is described more fully in our copending application directed to Method of Making Colored Mirrors, Serial No. 473,474, filed January 25, 1943 (now Patent No.

2,411,955), and in the following examples. We

' films involved in such structures and indicate how the composites of partially transparent reflective films cooperate together and. with the backing reflective mirror layer to give visually colored mirrors in which objects are clearly visible as images and in which the background cannot be viewed because of the opaqueness of the backing reflective mirror layer. It will be understood that in the figures like reference numbers indicate similar types of layers, or supports, and that the thicknesses as indicated by the drawings are solely for the purposes of illustration and that such thicknesses do not show the true proportions of the layers actually used in constructing our mirrors.

The sectional view of Figure 7 illustrates a second surfaced mirror made according to'our invention and which consists of the glass or other transparent support I, the single continuous semi-transparent mirror fllm 2, the substantially 15 opaque mirror backing 3, and the protective coating 4.

As some of our compound mirrors may be asily scratched or otherwise spoiled, we generally coat the back of the substantially opaque mirror backin with a protective layer of a paint, lacquer, or shellac, or we may copper-plate the backing or protect it with other means common to the mirror art. Such coatings do not take any part in the optical functioning of the mirror. We may also form our mirrors by disposing the semi-transparent film on the front surface of the support and the substantially opaque mirror film on the back surface of the support.

In Figure 7a the sectional view illustrates a first surface mirror made according to our invention which consists of the glass or other support 1-0., the semi-transparent mirror film 2-a, the substantially opaque mirror film 3a, and the thick protective transparent coating 4a. The latter coating is thicker than 9 quarter wave length factors to prevent the same having any light interference effect. In Figure 9 there is represented a second surfaced mirror structure which differs essentially from that of Figure 7 in that the composite, partially transparent light reflective film element 5 is used instead of the single film element 2. This .composite film element comprises the two partially transparent layers 6 and 1 which cooperate in producing interferenc coloration and in giving reflectivity.

Figure 9a illustrates a structure similar to that of Figure 9 but differing essentially in that the composite element 8 comprises the three partially transparent layers 9, I and H which cooperate in producing interference coloration and in giving reflectivity.

In Figure 10 there appears the first surface mirror structure comprising a support l which need not be transparent, the opaque mirror layer 3, and the partially transparent reflective layers 12 and i3 forming the composit element I 4. This composite element cooperates with the opaque mirror backing to determine the color and reflectivity of the mirror structure. The mirror may be protected on its front face by a transparent lacquer film I which may also be colored, if desired, such film being generally of a character similar to coating 4a as used in Figure 7a.

Figure a represents a complex mirror structure made in accordanc with the invention in which the contacting thin partially transparent layers are upon the opposite side of the support from the opaque reflective backing and in which a further single partially transparent layer is in contact with the backing. Thus, upon the front side of the transparent support I, the layers It and I! are contacting thin laminae which cooperate in giving interference effects, and upon the back of the support the layer I8 is the single layer of suitable properties and thickness to cause interference effects, and backing this is the opaque mirror reflective backing 3. The film 4 is merely a protective paint or other coating and the film i5 is of the transparent protective type as discussed in connection with Figure 10.

The mirror of Figure 11 comprises the glass support I, the three partially transparent layers I 9, and 2|, and the three partially transparent layers 22, 23 and 24. The layers are backed by an opaque mirror backing 3, and this in turn may be protected if desired by a paint or other coating 4. The several layers I9, 20, 2! may each be of similar thickness or varied and of the same or different material. The same may be true for the layers 22, 23 and 26. In such a structure the large number of semi-transparent layers act cooperatively together to form the composite element 25, and this composite element and the opaque mirror backing determine cooperatively the color and reflectivity of the mirror so produced.

While our compound mirrors receive their colors from the thinness of the reflective film or films and the resultant light interference, and from the nature and reflective character of this film or films, and further from the reflective nature of the substantially opaque backing layer, it is apparent that we can further modify the color range and reflectivities of our mirrors, if we use instead of colorless glass, as the mirror support, a colored glass or other colored support body of transparent material. The color absorption characteristics of the support will limit the total reflectivity percentage possible and shift the general tones of color in the direction of the color of the glass or support material used.

Similarly colored transparent layers thicker than and without light interference effect may also be introduced into the structures in any suitable arrangement.

While the necessary thickness in a single partially transparent film or layer when used alone in our mirrors with the backing mirror reflective layer needs to approximate a quarter wave length factor of or some simple multiple of this, or where multiple reflection of R times is possible, some submultiple such as to develop light interference colors, we find that, when employing a composite laminae of difierent substances to cooperatively produce light interference colors any one or more of these films may be of any extreme thinness, as long as they are continuous, but that any such film may not exceed a maximum thickness of if it is to function in part in determining or giving light interference colored reflected light or mirror images. Thus we have employed individual films in our cooperating laminae of partially transparent reflective layers which have ranged as low in thickness as the .004 part of a factor and as high as 4N to and we flnd films in the range of .01 to 2.5 times the quarter wave length factor in thickness to be particularly useful.

In the composite combinations of adjacent contacting partially transparent layers we may employ any combination of substances such as all the films being of metallic nature or of nonmetallic or of Organic nature, such for example, as adjacent thin copper and silver layers backed by an opaque aluminum mirror ora'djacent zinc sulfide and sodium fluoride layers; or these may be of mixed natures such as copper and magnesium fluoride or silica and copper. It will be apparent however that the order of arrangement of any given group of layers of different substances in the mirror with respect to the face of the mirror will result in mirrors of different color and reflectivity qualities as the layers are variously arranged with respect to each other.

Thus a second surface mirror having a fllm' of copper of .030 micron and transmitting 20% of light when deposited on glass gave a composite mirror, when further coated with a transparent layer of .080 micron thickness of magnesium fluoride and a final coating of opaque reflective metallic silver, which was of red purple red color and of 52% overall reflectivity. When a similar thickness of magnesium fluoride was flrst deposited upon the glass and this was then coated with the same .030 micron of copper and the latter then coated with the opaque silver layer, a second surface mirror of a brighter and whiter tone was secured, the reflectivity being higher (56%) and the degree of interference coloration being less.

Consideration of the various possible reflections at the different interfaces and of the internal multiple reflections of light within a composite layers, neglecting differences arising from absorption eflects. Where the adjacent contacting partially transparent layers are of different refractive index each film acts to retard the passage of a light wave through itself to a degree dependent upon its thickness and refractive index. Such retardations upon light rays such as a ray J in Figure B may act alone upon the rays dimirror structure, as diagrammatically shown by the light rays in Figure 8, shows that the light may follow many alternative courses to various degrees depending upon the total and selective light transmission character and action of the various films and the refractive indices of the adjacent layers and consequent reflective conditions at each of the interfaces, and as these rays are also influenced by the reflection character and conditions at the surface of the opaque refiector. Thus; in the mirrors just described the second surface mirror comprising the arrangement glass-copper-magnesium fluoride-opaque silver arranges these materials so that the refractive indices of the adjacent substances follow the sequence 1.56-(0.44 to 1.l0)--1.38-0.17 while in the second mirror arrangement glass-magnesium fluoride-copper-opaque silver the sequence of refractive indices of the adjacent layers is 1.56- 1.38-(.44 to 1.10)-0.17. It will be observed that the various differences in index of refraction of adjacent layers is generally lower for the second arrangement of the materials and consequently the relative reflection occurring at the various interfaces is less than in the flrst mirror construction and the interference action is less complicated and not as much light is apparently affected in such manner. While the adjacent layers may thus most effectively be arranged for maximum color developed by arranging the adjacent layers to show the most abrupt changes in index of refraction it is apparent that for the purposes of producing our mirrors most any arrangement will suflice and produce our desired color and reflectivity effects to some desired degree.

As previously indicated where the adjacent contacting partially transparent layers are of the same refractive index these layers cooperate in producing light interference in a manner Just as a single fllm of the total thickness of such various thicknesses of partially transparent coprectly reflected after passage through the top layer such as with ray L or multiply reflected ray N. Such rays can then cause interference with the ray K reflected directly from the top surface of the mirror. The retardations in the various contacting layers however are additive or cooperative in-effect with each other after passage of the light into the subsequent layers as would be true for such rays as M or P or the other multiply reflected rays such as O, T. and Q. It is apparent that each ray would show various and different retardations and that each may show interference with the front surface reflected ray K or similar rays or with others of the various rays such as those illustrated. The relative strength of the separate rays varies with the reflective conditions at the various interfaces and the light absorption effects experienced in passage of the rays through the layers it travels through.

Thus the conditions of interference and reflectionare extremely complex but the various films cooperate in securing the necessary retardation of some rays of light to get these out of phase with others and to thus cause the development of color by such light interference effects. .As previously indicated, color is developed by the various possible combinations of thicknesses among the separate partially transparent reflective films, each such film apparently cooperating with the other such films in producing the light interference. Thus in Figure 14 there is shown a chart which is generally illustrative of the means of making our colored mirrors and of the variation in color as the respective fllms forming the composite element are varied in thickness. This diagram is typical for our various mirrors and shows that colored mirrors resulting from various thicknesses of the two films used for example, vary in color continuously as either fllm is changed in thickness and further it isapparent from the general hyperbolic nature of these curves that the two materials cooperate or replace each other giving a color effect dependent upon some cooperative or summation effect of the two fllms. The diagram specifically shows the colors of second surface mirrors in which various thickness thin transparent copper films are deposited upon the glass and coated in one series of mirrors with silica films and in the other series of mirrors with magnesium fluoride fllms. In' each series the mirrors had an opaque silver mirror reflective backing. It is apparent that the silica and magnesium fluoride do not exert the same influence but that the effects by each are related in general to the relative refractive indices of each.

In Figure 14 the mirrors, formed with the per and silica layers as shown by the dotted lines or of copper and magnesium fluoride layers as shown by the full lines which fall upon one of the lines appearing in the diagram, are of the same color. This indicates that the widely varying sets of such films in varied thickness along such lines are cooperative together in general, each fllm acting to retard the passage of a light wave 19 through itself to a degree dependent upon its thickness and its refractive index, and the total retardation being some summation of such individual effects. That the two silver backed mirrors as shown in Figure 14 made with .037 micron copper film and a silica film of .040 and otherwise with a .0074 copper film and a silica film of .060 micron thicknessare both purples would seem to indicate that the relative changes in thickness of the two films had been compensating and that the light ray retardation effects in the two composite cooperating partially transparent laminae was much the same in view of the same general interference effect produced as indicated by the purple color in each case.

The purple color appearing in each case indicates clearly that the same wave lengths of light were altered by interference brought about by the retarded rays and that the spectral curve for each mirror would show a minimum at the same general wave length position. The spectral reflection curves would be expected to be generally similar for the two mirrors except as influenced by the selective absorption effects peculiar to the greater thickness of copper in the one case and the difierent relative reflection conditions at the different interfaces.

The curves of similar color as shown in Figure 14 are incomplete for all colors of the spectrum in each of the successive series but it is apparent that the spectral range was traversed at least once with the silver backed mirrors made with the transparent copper and silica films and that several spectral series followed each other in sequence with the mirrors made with the copper and magnesium films.

In view of the many complex paths as shown in Figure 8 in which the light rays may be directed within our multiple layered mirrors and of the considerable amount of and number of multiple internal reflections possible in the very thin highly or partially transparent layers it would appear that while the summation of the retardations of the light rays in the individual layers might be calculated by the well known vectorial method applied to such multiple layers it is apparent that due to the numerous multiple reflections occurring that the sums would not only indicate the cooperative films as acting similar to single films of thickness or some multiple of this as 3, 5, 7 but that interference color effects were secured in our mirrors at numerous thicknesses, such calculated totalized thicknesses as are submultiples of this such as and multiples of these figures by odd integers 1, 3, 5, '7 and so forth. In the examples which follow many cases appear where R appears to be of value 2, 4 and 8. However, as the phenomena in the mirrors as made under this invention are highly complex and further complicated by absorption effects we do not feel that such values as may so be calculated are limiting. As shown in Figure 14, any combinations of films, each sufiiciently thin, will when reflective mirror backed give us colored mirrors according to this invention.

The mirrors produced in the manner of this invention and in the examples which follow will have a desired color and reflectivity value and the color indicatedin each example will be the color of the mirror when viewing it directly at perpendicular or normal incidence. When viewing the mirror at different angles, the shade of the color will vary.

The nature of our new mirrors and their means of formation will be apparent from the following examples.

EXAIVIPLES 1 AND2 In these examples a single thin semi-transparent reflective film of lead sulfide is deposited by special chemical means, this film being of different thickness in each example and, in turn, in each case being backed with a relatively opaque silver mirror layer. Each of the mirrors is of diiferent color and spectral and total light refiective characteristics as shown in the respective spectral reflection curves given in Figures 12 and 13. For comparison the spectral reflection characteristics of a simple semi-transparent reflective lead sulfide mirror film alone of the same thickness is given in each figure so that it can be compared to the combination of such a mirror film with the backing layer of substantially opaque silver to demonstrate clearly the effects arising from the combination as compared to the semi-transparent layer alone.

Ordinary plate glass is thoroughly cleaned,

scrubbed with rouge and then rinsed thoroughly several times. The wet glass is then ready for mirroring. The mirroring is carried out at 68 degrees Fahrenheit and the solutions, glass and machines are all brought to this temperature by doing all the work in a constant temperature room regulated to this condition. This gives uniform conditions and with the mirroring solution used. the deposition proceeds at a constant rate so that the thickness of deposit is determined by the time the solution is permitted to act. Three aqueous solutions are made up for use as follows: Solution A, which contains 3.18% of sodium hydroxide and 0.00054% of sodium potassium tartrate. Solution B, which contains 3.7% of lead acetate and 0.264% of acetic acid. Solution C, .which contains 2.64 of thiourea. These three solutions are mixed together in equal quantities just prior to their being poured onto the glass. The mixed solution at the time of pouring is of 50 the following composition:

1.06% sodium hydroxide 1.23% lead acetate 0.88% thiourea 0.088% acetic acid 55 0.00018% sodium potassium tartrate As compared with the method of forming lead sulfide mirrors shown in the patent to Colbert et 2.1., 1,662,564, of March 13, 1928, it is seen that no we use a higher concentration of lead acetate and a lower concentration of thiourea. These changes have the effects of increasing to some degree the laying down of the nuclei uniformly and of slowing down the rate of reaction. These 5 efiects are also enhanced by the use of a temperature of 68 degrees Fahrenheit in contrast to the 95 degrees or higher ordinarily employed in depositing lead sulfide mirrors. However, these changes alone have been found to be insuflicient as it generally occurs, when mirror deposition is slowed down, that the securing of uniform deposits becomes more difilcult. As it is particularly necessary that the thin mirror films be extremely uniform because of their consequent 75 variat on in. color, if not, and also because of 21 their semi-transparent nature, we have found it necessary to add a new substance having a retarding effect on the deposition rate and one which facilitates very uniform deposition This substance is preferably sodium potassium tartrate. It is believed that the sodium potassium tartrate, which is used in very small amounts as it has a substantial eifect, operates by possibly decreasing greatly the rate of growth of lead sulfide on already deposited nuclei. Thus, greater numbers of nuclei are apparently caused to form and the growth on these nuclei becomes slow and uniform in every direction. While the depowere then flushed with considerable water and the surface thoroughly cleaned by gently rubbing with wet cotton.

Table 1 Total Film N o. of NMirrg; ggy; Color :3 :2" lflarcent Spectral Thick- FWatve RTimfis um r rans- Minimum ness, ac era a a Minutes flwflvuy mission Microns fleited 2a. 13. 3 Pale Y6ll0W 35. 7 41110 026 2% 1 4a.. 15. 2 Orange Yellow 31. 7' 4700 030 1 sition rate of the formula of Patent 1,662,564 can be decelerated by working even below 68 degrees Fahrenheit or by using less alkali, neither of these procedures will give satisfactory uniformity for the making of good interference colored mirrors. The use of the small amount of sodium potassium tartrate is thus very desirable, although we have found that other materials may v be used as retarders, as indicated in our said copending application.

In order to overcome the limiting of the amount of solution in contact with the glass at its edge by surface tension effects and the variation in deposit thickness at the edges, as a consequence, we find it peferable, in order to secure very uniform results, to place the wet glass to be mirrored in a stainless steel pan, precoated with lead sulfide, and to rock the pan about 35 times a minute using a metal frame insert in the bottom of the pan to keep the glass from shifting. Approximately 2.8 cc. of mixed solution per square inch of glass to be treated is poured over the glass in the tray and the rocking keeps this liquid uniformly flowing over the surface of the glass during the entire deposition With our new mixed solution, after about 8.5 minutes from the time of pouring, a darkening of the glass can first be noticed and the thickness of mirror film becomes progressively greater as the time increases. If the deposition is allowed to proceed for about 60 minutes, a completely opaque ordinary type lead sulfide mirror is secured, in which the thickness of coating is about 0.140 micron. Mirrors of this thickness with lead sulfide are usually laid down in about 7 minutes, using the solution of Patent No. 1,662,564, and these mirrors show no color, as indicated by the spectral reflection curve of Figure 1, and are The mirrors were then fiushed with considerable water and the surface thoroughly cleaned by gently rubbing with wet cotton. The wet glasses were then placed on a regular silvering table with the lead sulfide surface up and a silver mirror deposit was formed directly on top of the lead sulfide layer, using any of the regular chemical silvering formulae. For example, we may pour onto the glass, at about degrees Fahrenheit, a freshly mixed solution of 1.2% silver nitrate, 1.2% sodium hydroxide, 3.75% of concentrated ammonia water, and 0.20% invert sugar, all dissolved in water. After about ten minutes, the solution may be rinsed oil and the silver back formed may be rinsed thoroughly and dried, after which a protective coating of shellac may be applied on top of the silver. The semi-transparent lead sulfide coatings 2a and 4a were thus used, respectively, in forming the compound mirrors 3 and 6, given in Table 11 which follows.

In Table II, each of the mirrors is a compound colored mirror made with a single thin semitransparent lead sulfide reflecting film of increas ing thickness, going from Example 1, mirror 3, through Example 2, mirror 6, which film causes light interference effects, and an opaque silver mirror backing layer. This table shows the color of the mirrors, thus produced, and their total reflectivity value. Each of the mirrors was perfectly uniform in color and a good reflector. Comparison of the Examples 1 and 2 with the simple semi-transparent mirror films shown in Table I as samples 2a and 4a, which were used in making Examples 1 and 2, showed that by the use of the backing reflector, the colors were made deeper and the mirrors were mostly brighter, or as the two tables show the total percentage reflectivity was increased.

Table II Total Per- Lead Sulfide N o. of Example Color cent Reflecs Flim Thickwave Times Retivity ness, Microns Factors flected 1 a Blua,Yellow. aa a 4, 000 .026 1 2 0 Orange s5. 2 4, 800 .030 1 opaque. For the spectral reflectivity curve shown, the total reflectivity is 29%.

The spectral reflectivity curves Figures 12 and 13 respectively for these compound mirrors each shows a dip in the curve or minima. By comparison, the spectral reflectivity curve for an ordinary opaque film lead sulfide mirror, as shown in Figure 1, is free of any dip or indication of color. The minima in the spectral reflectivity curves shows the light rays which are being diminished in the reflected light by interference. As would be expected for interference effects, the minima continually shift in the examples toward the longer red rays, as the fllm thickness of the semi-transparent lead sulfide layer, in the examples, is increased. The wave length at which the minima appear in the spectral reflection curves for the various samples, in both Tables I and H, are shown as well as the thickness of the lead sulfide layer.

In Examples 1 and 2, the semi-transparent lead sulfide reflective layer thickness, at which the interference minima occur with the difierent light waves, is related to the wave length by the ratio of The apparent color of these mirrors is obviously largely dependent upon the color of the light removed by interference.

In order to more clearly bring out the nature of the color and reflective characteristics of our new compound mirrors and how the backing layer 01' an opaque mirror, such as silver, plays a part in determining these characteristics, the spectral reflectivity curves in Figures 12 and 13 are given. In each figure, the reflectivity curve for one of our new compound mirrors is given and compared with the spectral reflectivity curve for an equally thick semi-transparent lead sulflde reflective film which has not been backed up with silver. Thus, in Figure 12 the film 2a of Table I is compared with our Example 1. Each curve shows the dip in its curve at the same place as caused by interference. The silver-backed example has, however, a higher reflection in the green and a higher total reflection. In Figure 13, the film 4a is compared with Example 2 and a much higher reflectivity in the yellow, orange and red is to be noted with the silver-backed mirror and the total reflectivity is 35.2%, as compared with 31.7%.

It is quite obvious from the curves of Figures 12 and 13 that the films of lead sulfide are not only quite transparent in the thicknesses usedin our mirrors, as shown in Table I, but that this particular material in these thicknesses is proportionally more transparent in the reds and, as a result, backing up these fllms with a material reflecting in the red, such as silver, brings about considerable changes in the red reflectivity, increasing this in some cases and very definitely decreasing this when the lead sulfide layer is of the right thickness to cause interference losses in the red.

The mirrors of Examples 1 and 2 may be used in furniture, in store decoration, and ma wide variety of decorative and reflector uses.

EXAMPLES If in preparing the semi-transparent lead sulflde reflective coating for Example 1, we arrange to blow a fine gentle current of air on the top of the glass plate, while it is in the pan being coated with the lead sulflde, and do so at several points, the resulting mirror produced when backed with a coating of silver will not be of one uniform color throughout but will show a variegated pattern in various colors. This is due to the fact that the fllm thickness of the deposited lead sulflde at various points on the glass will vary and the eflects of the backing layer of silver will also vary at the diilerent spots. Such a coating may 0bviously be employed as one 01' the layers in the composite film means such as the lead sulphide layer disclosed herein.

EXAMPLES 4 TO 14 Cleaned pieces of glass were mounted within a chamber and, in the high vacuum created therein, there was deposited by thermal evaporation upon the glass a thin layer 01' copper of a thickness in each speciflc example a shown in Table III. Upon this continuous thin layer, which showed the transparency as indicated in the table, there was next deposited by thermal evaporation a layer of magnesium fluoride of th respective thicknesses indicated and thereafter in each case the mirror was completed by further depositing on the magnesium fluoride an opaque reflective backing of silver. Silver has a refractive index of 0.17.

Each of the second surface mirrors thus employs a composite fllm element of copper and magnesium fluoride and a backing of opaque silver and in each case the mirror produced showed a visually eiiective color and clear reflected images of high brilliancy. The colors and total reflectivity of each of the mirrors is given in the Table III, the colors being expressed according to theIMunsell color system in these ex- Table III Tm kn in Thickness in Percent Percent Tom] g: L Properties 01 Completed Mirror Transmis- Ransom-vi; 4N Example No. (sjion of or Coppery opper Layer Layer Copper MgF|' Copper MgFa S ctral C Total Re- Layer Layer yer Layer ma flectivity 22 27 (B0 (B0 21 84 6, 250 RPR 46 22. 27 .030 .107 .17 1.07 6, 250-5,500 RPR 39 22 27 030 10 1.04 6, 850 B 35 22 27 .030 .159 .25 1.86 4,700 YRY 60 22 27 0B0 25 2 28 4, 700 YRY 60 67 9 .010 .130 .07 .84 5,250 RPR 33 57 9 010 107 035 86 6, 850 B 43 67 9 .010 .130 .(B 1.50 4,000-4,200 GYG 68 74 6 .005 .130 018 1.00 7, 200 BG 45 13 .015 .130 .088 1.30 5,500 BR 7.8 42 .050 .130 .176 .98 7,300 G 

